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Abstract:

The present invention relates to aluminum-lithium alloys in general and,
in particular, such products as used in the aircraft industry and the
welding of these.

Claims:

1. Process for preparing an aluminum lithium alloy product to be
fusion-welded comprising: (i) procuring a hot-worked aluminum alloy
product including at least 0.8% of lithium by weight, (ii) optionally
cold-working the product so obtained, (iii) cleaning at least one surface
to be welded of the product so obtained, (iv) covering at least one
cleaned surface of the product so obtained with a coating whose
characteristics when dry are a quantity ranging from 0.1 to 5 mg/cm2
and a fluorine concentration of at least 10% by weight, (v) performing a
solution heat-treatment at a temperature greater than approximately
450.degree. C. followed by quenching of the product so obtained.

2. Process according to the claim 1 in which said aluminum lithium alloy
comprises at least 1.4% of lithium by weight.

3. Process according to claim 2 in which said aluminum lithium alloy is
selected from the group consisting of alloys 2090, 2091, 2196, 2097,
2197, 2297, 2397, 2099, 2199, 8090, 8091, and 8093.

4. Process according to claim 1 in which the hot-worked and optionally
cold-worked product is a flat-rolled or extruded product with thickness
lower than 5 mm.

5. Process according to claim 1 in which the cleaning of (iii) is carried
out by treating with an aqueous solution with a pH greater than 9.

6. Process according to claim 1 in which (iii) and (iv) are carried out
on the majority of, or optionally on the whole, surface of said product.

7. Process according to claim 1 in which said coating comprises a binder
whose concentration when dry ranges from 5% to 50% by weight.

8. Process according to claim 1 in which said coating comprises when dry,
as a percentage by weight, from 75% to 95% of NaBF4, from 0 to 15% by
weight of carboxymethyl cellulose and from 0 to 15% of a silane.

9. Process according to claim 1 in which said coating comprises after
drying, as a percentage by weight, from 50% to 100% of KxAlyFz, from 0 to
5% by weight of CsxAlyFz and from 0 to 50% of a binder.

10. Process according to claim 9 in which said binder is an alkyl
silicone resin.

11. Process according to claim 9 in which said coating is deposited by
electrostatic powdering.

12. Fusion-welded assembly between a first aluminum alloy member
comprising at least 1.4% of lithium by weight and at least one second
metal alloy member, said first member being a flat-rolled or extruded
product of a thickness less than 5 mm, the first member having been
prepared by the process according to claim 1 wherein a weld between said
members is substantially free from porosities.

13. Welded assembly according to claim 12 in which the thickness
tolerance of said first member is plus or minus 0.20 mm.

14. Welded assembly according to claim 12 including at least one second
aluminum alloy member including at least 0.8% of lithium by weight.

15. Welded assembly according to claim 12 in which said first member is
an extruded section and said second member is a sheet or an extruded
section.

16. Fuselage panel including a welded assembly according to claim 14.

17. Welded assembly according to claim 12 in which the thickness
tolerance of said first member is plus or minus 0.15 mm.

18. Welded assembly according to claim 12 in which the thickness
tolerance of said first member is plus or minus 0.10 mm.

19. Welded assembly of claim 12, wherein said first member comprises a
flat-rolled or extruded product of a thickness less than 2 mm.

Description:

SCOPE OF THE INVENTION

[0001] The present invention relates to aluminum-lithium alloys in general
and, in particular, such products as used in the aircraft industry and
the welding of these.

BACKGROUND OF RELATED ART

[0002] Aluminum-lithium alloys (Al--Li) have long been recognized as an
effective solution for reducing the weight of structural elements because
of their low density. Their use is systematically considered for the most
modern aeronautical structures. In addition, the use of welding instead
of the usual techniques of riveting is also a current trend in the field
of aeronautical engineering. It therefore goes without saying that in
order to be used in aeronautical engineering, Al--Li alloys must
preferably be able to be fusion-welded without difficulty.

[0003] U.S. Pat. No. 5,032,359 describes a family of weldable Al--Li
alloys, the Aluminum-Copper-Lithium-Magnesium-Silver alloys. These alloys
are also known under the trade name of "WELDALITE®" which
particularly stresses their weldability. However, it was recognized in
this initial patent and the later literature that this type of alloy was
sensitive to the formation of porosities during welding. The mechanism
behind this problem is poorly understood; it seems specific to Al--Li
alloys.

[0004] It is known from the ASM Handbook "Aluminum", 1991 pages 402-403
that a solution to this problem can be provided by carrying out
preprocessing, typically surface etching of about 250 μm, just before
welding. The porosities after welding are then observed to disappear.
However this practice has several disadvantages: it requires a fairly
long surface treatment stage before welding which complicates the
manufacturing process and can lead to prohibitive investment in plants
which are not equipped with surface treatment lines.

[0005] This treatment can prove to be difficult to perform homogeneously,
in particular on extruded sections of complex shape. In addition,
chemical etching of about 250 μm is difficult to perform accurately
for thin parts, typically about 1 to 2 mm: etching on both faces may
account for approximately 25 to 50% of the final thickness which presents
technical problems for respecting thickness tolerances and must be taken
into account for dimensioning the parts. Finally, this treatment causes a
metal loss which is economically very unfavorable, in particular for thin
parts of low thickness. The document Ellis M B D "Fusion welding of
aluminum-lithium alloys" Welding and Metal Manufacture, IPC LTD. HAYWARDS
HEATH, GB vol 64, n° 2, Feb. 1, 1996 page 44/56, 58, 60 also
states that it is necessary to remove 0.2 mm from each face to carry out
welding free from porosities on aluminum-lithium alloys. The document
Ryazantsev V I "Preparation of the surface of aluminum alloys for arc
welding" Welding International, Taylor & Francis Abingdon, GB, vol. 16,
n° 9 Jan. 1, 2002 pages 744-749, describe various methods of
preparing products made of aluminum alloy, in particular aluminum-lithium
alloy, including stages of degreasing and chemical etching and solution
heat treatment in a vacuum oven.

[0006] U.S. Pat. No. 6,881,491 describes a process for protecting an
aluminum surface able to be coated in order to avoid blistering during
heat treatment. This process is not intended for surface preparation
before welding of aluminum lithium alloys.

[0007] In the same way, patent application EP-A-0 882.809 indicates
treatments containing small quantities of fluorine to prevent oxidation,
but does not reveal their use before welding or for aluminum-lithium
alloys.

[0008] There therefore exists a need for a process for preparing aluminum
lithium alloy parts for being welded, which makes it possible to avoid
the formation of porosities in welds while avoiding all the disadvantages
related to surface etching.

SUBJECT OF THE INVENTION

[0009] The subject of the invention is a process for preparing an aluminum
lithium alloy product for it to be fusion-welded, including the
successive stages of:

(i) procuring a hot-worked aluminum alloy product including at least 0.8%
of lithium by weight, (ii) optionally cold-working the product so
obtained, (iii) cleaning at least one surface to be welded of the product
so obtained, (iv) covering at least one cleaned surface of the product so
obtained with a coating whose characteristics when dry are a quantity
ranging between 0.1 and 5 mg/cm2 and preferably between 0.5 and 4 mg/cm2,
and a fluorine concentration of at least 10% by weight, (v) performing a
solution heat-treatment at a temperature greater than approximately
450° C. followed by quenching of the product so obtained.

[0010] Another subject of the invention is a fusion-welded assembly
between a first aluminum alloy member including at least 1.4% of lithium
by weight and at least one second metal alloy member, the first member
being a flat-rolled or extruded product of thickness less than 5 mm and
preferably less than 2 mm, the first member having been prepared by the
process according to the invention, characterized in that the weld is
substantially free from porosities.

[0011] Still another subject of the invention is a fuselage panel
including an assembly welded according to the invention.

DESCRIPTION OF THE FIGURES

[0012] FIG. 1: classification of quality in terms of porosity of the
welded joints

[0013] FIG. 2: profiles used for the tests

DETAILED DESCRIPTION OF THE INVENTION

[0014] Unless otherwise stated, all the indications concerning the
chemical composition of the alloys are expressed as a percentage by
weight based on the total weight of the alloy. The designation of alloys
is compliant with the rules of The Aluminium Association, known to
experts in the field. The definitions of metallurgical states are
indicated in European standard EN515. Unless otherwise stated, the
definitions of standard EN 12258-1 apply. The thicknesses of extruded
products are defined according to standard EN2066.

[0015] Dry coating is taken to mean the state reached by the coating when
it is dry throughout its thickness as defined by standard ISO 9117-90,
which is different from a coating that is dry on the surface while the
great majority of the coating has not yet stabilized.

[0016] The process according to the present invention is a process for
preparing an aluminum lithium alloy product to be fusion-welded. Fusion
welding is taken to mean processes, such as spot welding, flash welding,
laser welding, arc welding, electron beam welding, in which welding is
carried out above the melting point of the aluminum-lithium alloy, in the
liquid phase. Within the framework of this invention, aluminum-lithium
alloy is taken to mean alloys including at least 0.8% of lithium by
weight. The process according to the invention is particularly
advantageous for alloys including at least 1.4% of lithium by weight.
Advantageously the process according to the present invention is applied
to an alloy product selected from the group made up of alloys 2090, 2091,
2196, 2097, 2197, 2297, 2397, 2099, 2199, 8090, 8091, 8093. In a
preferred embodiment, the process according to the invention is applied
to an alloy 2196 product.

[0017] In the first stage, a hot-worked aluminum lithium alloy product is
procured. Hot-working is taken to mean an operation for deforming a block
obtained for example by semi-continuous casting. Hot-working of aluminum
lithium alloys is carried out typically at an initial welding temperature
greater than 350° C. or 400° C. Hot-working operations are
typically rolling, extruding and forging. In a preferred embodiment of
the invention, the hot-working operation is hot extruded.

[0018] In a second optional stage the hot-worked product can then be
cold-worked in order to obtain a thinner product. Cold-worked operations
are for example cold rolling, drawing and/or hammering.

[0019] At least one surface intended to be welded of the hot-worked and
optionally cold-worked product is then cleaned. The purpose of cleaning
is to eliminate the main residues from the hot-working stages. These
residues are primarily hot-working oils and particles: oxides and/or
metal particles. Cleaning can be carried out by any means suitable for
eliminating these residues. Cleaning is not surface etching, so the
thickness of metal eliminated during cleaning is less than 20 μm per
side and preferably less than 10 μm per side. Chemical cleaning is
typically carried out using an organic solvent such as for example an
alcohol, a ketone or an alkane, or an alkaline grease-remover such as for
example a grease-remover containing soda, potash or sodium carbonate, or
an acid grease-remover, for example a grease-remover containing chromic
acid, sulfuric acid or phosphoric acid. Several types of cleaning can be
combined. However the present inventors noted that the combination of
various cleaning operations, for example the combination of an alkaline
cleaning and an acid cleaning, did not provide any technical advantage
and uselessly complicated the process. Preferably cleaning is therefore
carried out using one only family: cleaning by organic solvent or
cleaning by alkaline degreasing or cleaning by acid degreasing. In an
advantageous embodiment of the invention, cleaning is carried out by
treatment with an aqueous solution with a pH greater than 9. Cleaning can
be followed by surface rinsing, for example with demineralized water.

[0020] The surface so cleaned is then covered with a coating whose
characteristics when dry are a quantity ranging between 0.1 and 5 mg/m2
and preferably between 0.5 and 4 mg/cm2, and a fluorine concentration of
at least 10% by weight, and preferably of at least 25% by weight.

[0021] The coating can be deposited in the form of a solution, typically
by immersion or spraying or in the form of powder, typically by
electrostatic powdering. In the case of depositing a solution, the
solvent is then evaporated by any suitable means in order to obtain a dry
coating. In the case of a deposit in the form of powder, a dry coating
can be obtained directly. Many fluorinated substances can be used to
reach the desired fluorine concentration in the dry coating.
Advantageously, the fluorine is in the form of a metal fluorine salt or a
fluorinated compound. Useful examples of salts within the framework of
the invention are given in Table 1. Fluxes of the Nocolok® brand can
be used advantageously. Most are products containing aluminum and
potassium fluoride with the general formula KxAlyF.sub.z
possibly containing various additives.

[0022] Preference is given to products that at least partially decompose
during the solution heat treatment in order not to hinder the fusion
welding. However, the present inventors noted that the residues of
certain fluorinated substances, and in particular substances containing
potassium and aluminum fluoride did not cause any difficulty during
fusion welding, probably because of their melting point lower than the
melting point of aluminum alloys. The addition of a cesium and aluminum
fluoride is advantageous.

[0023] Advantageously, the coating comprises a binder whose concentration
when dry lies between 5% and 50% by weight. The binder makes it possible
to obtain a homogeneous and reproducible deposit of the fluorinated
substance. For coatings deposited from an aqueous solution, a thickener
is typically used, such as for example those used in the food, cosmetic
or painting industry. Carboxymethyl cellulose is a useful binder within
the framework of the invention. A coupling agent can also advantageously
be used in coatings deposited from an aqueous solution. Of the coupling
agents, silanes are particularly advantageous. According to the
invention, silane can be any silane of general formula R'Si (OR)3, where
R' is a group containing at least one organic radical and where OR is an
alkoxy radical. Preferably, an aminosilane or an epoxy silane is used,
such as, for example, the silanes AMEO (3-aminopropyltriethoxysilane) or
Glymo: (3-glycidopropyltrimethoxysilane). For coatings deposited in
powder form, the binder is typically a polymeric compound. Useful
polymers within the framework of the invention comprise epoxy resins,
polyurethane resins, polyolefin resins, polyacrylate resins, polyester
resins, latexes, alkyl silicone resins and polyisocyanate resins. The use
of alkyl silicone resins is preferred.

[0024] In one embodiment of the invention the coating is deposited from an
aqueous solution and comprises when dry, as a percentage by weight,
between 75% and 95% of NaBF4, between 0 and 15% of carboxymethyl
cellulose and between 0 and 15% of a silane. Advantageously, the coating
according to this first embodiment does not comprise any other compounds
than NaBF4, carboxymethyl cellulose and silane. In a second embodiment of
the invention, the dry coating deposited preferably by electrostatic
powdering comprises, as a percentage by weight, between 50% and 100% of
KXAlYF.sub.Z, between 0 and 5% of CsxAlyF and between
0 and 50% by weight of a binder, preferably an alkyl silicone resin.
Advantageously, the coating according to this first embodiment does not
comprise any other compound than KXAlYF.sub.Z,
CsxAlyF, a binder.

[0025] Cleaning and coating deposit are not necessary over all the surface
of the product because the invention relates to the improvement of the
quality of the welded joint and only surfaces intended to be welded
therefore require treatment. However, it may be advantageous to carry out
these cleaning and deposit stages on the majority or preferably on all
the surface of the product, because this has an advantage in terms of
simplicity and reproducibility of the treatment.

[0026] After depositing the coating, solution heat-treatment is performed
at a temperature higher than approximately 450° C. followed by
quenching. This is a conventional operation on aluminum-lithium alloys
which is carried out in ambient air or in a more slightly oxidizing
atmosphere such as one including argon, helium, C02, nitrogen, alone or
in a mixture. An advantage of this invention is to obtain welds without
porosities whatever the atmosphere used during the solution
heat-treatment. The invention may also be advantageous if one carries out
intermediate softening heat treatments at a temperature higher than
250° C. or 300° C. during the stages of cold working, for
example, between cold rolling runs for sheets or drawing for tubes.
Optionally, at least one surface covered with a coating of the product
placed in the solution obtained in this way is cleaned. During the
solution heat-treatment and quenching stages the coating is at least
partially removed. In certain cases, residues of the coating remain
present on the surface. These residues may give an undesirable appearance
to the product and/or prove to be awkward during welding operations. They
may, if necessary be cleaned, the conditions of cleaning already
described being suitable. On this point, the coatings including a
fluorinated substance of the NaBF4 or KBF4 type are advantageous because
in the conditions of the invention, no visually apparent residues remain
after solution heat-treatment and quenching. The coatings including a
fluorinated substance of type KxAlyF.sub.z and/or
CsxAlyF.sub.z are also advantageous because in the residues
which remain after solution heat-treatment and quenching the welding
operations do not hinder fusion.

[0027] After or before the optional cleaning stage which follows the
solution heat-treatment, cold working and/or leveling and/or
straightening and/or forming and/or aging usual for this type of product
can if necessary be performed. The product resulting from the preparing
process according to the invention is ready to be fusion-welded. Fusion
welding is carried out using any fusion welding technique. In one
embodiment of the invention, fusion welding is performed by laser welding
in an inert atmosphere. The preparation made confers great stability on
the aluminum-lithium alloy product. Fusion welding can if necessary be
carried out several weeks after the end of the treatment. Preparation of
the Product According to the Invention Means that Fusion Welds
substantially free from porosities are obtained.

[0028] As it is not well understood why welds made on aluminum-lithium
alloys have a high propensity to form porosities, the mechanism
explaining the effectiveness of the treatment according to the invention
is particularly difficult to elucidate. Without being tied to any
particular theory, the present inventors believe that the coating
interacts synergistically with hydrogen in the atmosphere and lithium in
the alloy, during solution heat-treatment.

[0029] The invention is particularly advantageous when the hot-worked and
optionally cold-worked product is a flat-rolled or extruded product with
thickness lower than 5 mm and preferably lower than 2 mm. The thinner the
product, the trickier it is to carry out the known process of etching
before welding reliably, and the harder it becomes to respect the
thickness tolerances of the product.

[0030] The invention therefore makes it possible to manufacture a fusion
welded assembly, with a weld substantially free from porosities between a
first aluminum alloy member including at least 1.4% of lithium by weight
and at least one second metal alloy member, in which the first member is
a flat-rolled or extruded product of thickness less than 5 mm and
preferably less than 2 mm, the first member having been prepared by the
process according to the invention. Advantageously, the thickness
tolerance of the first member is plus or minus 0.20 mm, preferably plus
or minus 0.15 mm and preferably still plus or minus 0.10 mm. The first
member and the second member are worked products, typically a extruded
section, a sheet, a tube, a bar or a forged part. The possibility of
obtaining such a thickness tolerance, in particular for products of low
thickness, is a technical advantage of the invention because with
processes according to prior art, using chemical etching of 0.2 mm to
0.25 mm on each face which can account for approximately 25 to 50% of the
final thickness of the product, it is difficult to obtain such
tolerances. The invention is particularly advantageous when the two
members of the welded assembly are made of aluminum-lithium alloy, as it
is more difficult in this case to obtain welds substantially free from
porosities. In an advantageous embodiment of the invention, the welded
joint comprises at least one second aluminum alloy member including at
least 0.8% of lithium by weight.

[0031] In another embodiment of the invention, the second member is a
titanium alloy and the assembly is preferably a "welding-brazing"
operation in which the aluminum lithium alloy member undergoes fusion but
not the titanium alloy member.

[0032] In still another embodiment, the second member is, in any product,
weldable by fusion with the first member, in particular any aluminum
alloy.

[0033] In an advantageous embodiment of the invention the first member is
a extruded section, preferably made of alloy 2196 and the second member
is a sheet or a extruded section. Assemblies welded according to the
invention find particularly advantageous applications in aeronautical
engineering with regard to the manufacture of structural elements. The
term "structural element" refers to an element used in mechanical
engineering for which the mechanical, static and/or dynamic
characteristics are of particular importance for the performance and the
integrity of the structure, and for which a structural analysis is
generally prescribed or carried out. These are typically mechanical parts
the failure of which is likely to endanger the safety of said
construction, its users or others. For an aircraft, these structural
elements comprise the parts which make up the fuselage (such as the
fuselage skin, stringers, bulkheads, circumferential frames), the wings
(such as the wing skin, stringers or stiffeners, ribs and spars) and the
tail unit, made up of horizontal and vertical stabilizers, as well as
floor beams, seat tracks and doors.

[0034] In a preferred embodiment, the assemblies welded according to the
invention are used for the manufacture of fuselage panels.

EXAMPLES

Example 1

[0035] In this example, extruded sections made of alloy AA2196, thickness
1.6 mm and 3.2 mm in state T4 were fusion welded. The extruded sections
of thickness 1.6 mm and thickness 3.2 mm are shown in FIG. 2. Solution
heat-treatment was 45 minutes at 524° C. The welding lines were
made by laser welding with a filler wire made of alloy 4047, a power
level of 2300W and a welding speed of 5.4 m/min, in an atmosphere made up
of a mixture of Ar (30%) and He (70%). Etching with a controlled
thickness ranging between 0 and 300 μm per face was carried out using
an alkaline etching solution.

[0036] The presence of porosities in the welds obtained was characterized
by x-ray imagery. FIG. 1 illustrates 4 levels of porosities used to
evaluate the results obtained. Level A corresponds to the presence of at
the most a very low number of pores, the welding is substantially free
from porosities, and weld quality is good. Level B corresponds to a
higher pore density than that of level A, the pore diameter remaining
lower than 0.5 mm. Level C corresponds to a still higher density than
that of level B, the pore diameter remaining lower than 1.5 mm. Level D
corresponds to a high pore density, certain pores having a diameter
greater than 1.5 mm.

[0037] The results obtained are presented in table 2. Etching of 200 μm
to 250 μm proves to be necessary to obtain welded assemblies
substantially free from porosities.

[0038] In this example, extruded sections made of alloy AA2196, thickness
1.6 mm the cross-section of which is described in FIG. 2 were coated with
the products indicated in table 3, after cleaning in an alkaline medium
followed by rinsing in de-ionized water and neutralization treatment in
58% by volume nitric acid for 1 minute and rinsing in de-ionized water.
After drying the coating, the amount deposited was measured and the
extruded sections underwent solution heat treatment and were quenched
before undergoing welding lines in conditions identical to those of
example 1. The results in terms of the quality of the welded assembly are
also indicated in table 3.

[0039] Compared to the AA1 and AA2 reference samples, the majority of the
samples tested show an improvement in the porosity of the welded
assemblies apart from those samples for which the active substance is
TiB2. The samples treated with B2O3 have many surface residues after
solution heat-treatment.

Example 3

[0040] Extruded sections of thickness 1.6 mm made of alloy AA2196, of
cross-section identical to that of the preceding examples were obtained
by casting billets the composition of which is supplied and extruding at
a temperature greater than 400° C. Surface preparation treatments
before solution heat-treatment were carried out. First of all the
extruded sections were cleaned in an alkaline solution, followed in
certain cases by treatment in an acid solution. Three types of treatments
were then carried out: a first treatment containing sodium fluoroborate
(NaBF4), a second treatment containing boron oxide and a third treatment
based on aluminum and potassium fluoride (KXAlYFZ). The KXAlYFZ treatment
contained the flux referenced by Nocolok®Cs FLUX®, this flow
containing between 95 and 100% of aluminum and potassium fluoride K2AlF5
and less than 5% of cesium fluoroaluminate CSAlF4. A polymethylsiloxane
resin SILRES® MK powder by Wacker Chimie was added in certain
compositions.

[0041] Glymo silane (glycidopropyltrimethoxysilane) was added to the first
treatment. In addition, various solution heat-treatment conditions were
used. Two furnace atmospheres were tested: a standard atmosphere and a
deliberately humidified atmosphere, in order to create more severe
conditions.

[0042] The conditions used and the results obtained are given in Table 4.

[0043] In the absence of surface treatment before solution heat-treatment,
porosities are present in all cases.

[0044] The NaBF4 treatment makes it possible to obtain satisfactory
results in the majority of cases. Only the most severe conditions (45 min
524° C.-humid air) lead to a level C porosity density. It is also
to be noted that acid treatment after the cleaning operation in an
alkaline medium does not provide any advantage, exactly identical results
being obtained with or without this additional treatment. The B203
treatment did not make it possible to obtain favorable results
homogeneously and reproducibly. For this reason, several levels of
porosity density observed locally have been indicated. In addition, many
residues are to be observed on the surface after the stages of solution
heat-treatment and quenching. Additional cleaning (alkaline cleaning and
acid treatment) of the surface after solution heat-treatment and
quenching and before welding makes it possible to eliminate the majority
of these residues and then an improvement in the porosity density is to
be observed, without however reaching an acceptable level A quality
homogeneously and reproducibly. The KXAlYFZ treatment gave excellent
results (level A) for all the conditions of solution heat-treatment
tested. In addition, the absence of residues detrimental for welding on
the surface means that it is not necessary to carry out cleaning
treatment after solution heat-treatment.